Photoelasticity is well known as a technique for stress and strain analysis. The photoelastic method is based upon the unique property of some transparent material, particularly certain plastics that enable the use of structural members made of such materials to visually display stress patterns in the member. When a photoelastic member is stressed and a ray of polarized white light enters the member, the white light is believed to be divided into two component waves, each with its plane of polarization parallel to one of the principal planes of stress and strain. The white light travels along these two planes with different velocities depending upon the magnitude of the stress in the material. The light waves then emerge from the model in two planes with a phase difference so that when these waves are brought together at a polarizing panel, which functions to bring the waves into a common plane so that optical interference can take place, the resulting optical interference shows up as a "rainbow of colors" on the model, i.e., a pattern of colored bands or isochromatic fringes.
Heretofore, these stress or strain patterns of a rainbow of colors were used to analyze structural strength in a model of the structure. I have discovered that by placing a polarizing filter in front of a camera lens and a sheet of plastic material which has been stressed in front of the polarizing filter, I can produce, at the image plane behind the lens, color pictures with a "rainbow of colors" superimposed thereon.